1. Field of the Invention
Exemplary embodiments of the present invention relate to a dimmer for a light-emitting device and, more particularly, to a dimmer for a light emitting device, which provides a dimming function for a light emitting device by switching an alternating current (AC) input voltage at a high speed under pulse width modulation control to adjust the root-mean-square (RMS) value of the AC input voltage.
2. Discussion of the Background
In general, a lamp dimming function allows a user to control brightness of the lamp but may be restrictively used in practice. Currently, energy conservation has become an important concern in association with an increase in electrical energy consumption. Accordingly, the lamp dimming function has become a significant way to conserve energy rather than an optional function for user convenience. Further, a light-emitting diode (LED) has attracted attention as an environmentally friendly light source capable of improving energy conservation.
A conventional representative dimmer dims light from an AC LED by adjusting the root-mean-square (RMS) value (Vrms) of AC voltage by controlling the AC phase of the AC voltage using a semiconductor device, such as a triode for alternating current (Triac).
FIG. 1 is a block diagram of a conventional dimmer using a Triac. Referring to FIG. 1, the dimmer 10 includes a Triac switch 14 and an R/C (resistor/capacitor) phase controller 16. The Triac switch 14 supplies or blocks AC voltage from an AC voltage source 12 to a lamp, i.e. an AC LED 18. The R/C phase controller 16 includes a resistor R and a capacitor C to drive the Triac switch 14 by generating a phase control signal, that is, a gate turn-on signal, when an AC input voltage is 0 V. The phase control signal is an AC voltage signal delayed by a time constant determined by the resistor and capacitor of the R/C phase controller 16. The Triac switch 14 is turned on by the gate turn-on signal from the R/C phase controller 16 to allow the AC voltage to be supplied to the AC LED 18.
Thus, upper and lower dimming ranges of the Triac dimmer may be limited depending on the drive voltage of the Triac switch 14 and the operating characteristics of the resistor and capacitor of the R/C phase controller 16, thereby causing the AC LED to flicker. Further, in the Triac dimmer, the Triac switch 14 is abruptly switched by the gate turn-on signal output from the R/C phase controller 16, which may cause excessive generation of harmonics during the switching process.
In a phase control scheme of the Triac dimmer, the AC input voltage serves as a very important parameter in determining an output voltage and may not be a constant value in actual practice. A commercial AC power system creates various forms of loads, which may cause the system voltage to vary 10˜20% depending on load conditions. Therefore, although the Triac dimmer has a fixed phase angle which determines the dimming range, an output voltage corresponding the AC voltage may vary at a constant ratio. Accordingly, the variation in output voltage may cause the AC LED to flicker.
Therefore, there is a need for a new type of drive circuit and control circuit for an AC voltage source in order to obtain a wider dimming range and a linear dimming function.